1. Field of the Invention
The invention concerns an optical device and its application to generate an adjustable wave front deformation of a laser beam. Using such a device aims at modifications of beam spot geometry in the focal range of the laser beam and with it modifications of the beam parameter product of the laser beam.
2. Brief Description of the Related Art
Using laser beaming, for example for material processing, it is often desirable to have the option to use a special beam spot geometry to optimize the work results. A laser beam focus or a beam spot with ring-shaped intensity distribution is required for many applications.
The use of a so-called axicon, for example, is mentioned in the State of the art, that i.e. a lens with a cone-shaped effective surface which is inserted into the optical path of laser processing optics. This type of devices is described in documents DE 28 21 883, DE 100 44 522, DE 102 54 847 or DE 10 2010 016 628.
The devices described in the documents mentioned above aim at a ring-shaped beam distribution on at least one plane. The mentioned devices cannot be used to adjust the beam parameter product continuously, for example by adjusting the diameter of the ring. Modifications of the diameter of the ring require replacing the axicon with another one with a different cone angle of the cone surface.
It is mentioned in the State of the art that it is necessary to place two optical elements with respectively specially shaped surfaces in the optical path to generate variable, i.e. adjustable wave front deformations. The optical elements are positioned transverse to the beam direction and they are displaceable against each other. It is possible to generate different types of wave front deformation depending on the type of optical elements. The lateral displacement of elements with spherical surfaces of which the refractive powers compensate each other allows inclining a plane wave front by an adjustable angle. Thus, the device acts as an optical wedge with an adjustable wedge angle. Such a device is called “Abat's wedge” in the field of technical optics. A similar device of this type, however using elements with cylindrical surfaces, is mentioned in DE 199 61 918.
A similar device, which is described in U.S. Pat. No. 3,305,294, is used to generate a parabolic wave front deformation with an adjustable curvature, which corresponds to a lens with adjustable refractive power or with an adjustable focal length regarding its effect. In this case, the elements have to consist of surface shapes of which the local thickness of the elements is mainly defined by cubic terms, i.e. power functions of third degree with the lateral coordinates x and y in order to have the requested optical effect. This type of device has meanwhile been called “Alvarez lens” because it was invented by Alvarez. This lens with variable refractive power is used, for example, to adjust continuously the focal position of a lens. However, it is not possible to modify the beam parameter product.
None of the mentioned devices is able to generate a wave front deformation by means of which the beam parameter product of the laser beam can continuously be adjusted and magnified beginning with its original value.